Optical waveguide

ABSTRACT

Disclosed is an optical waveguide having an optical confinement layer.

TECHNICAL FIELD

[0001] This invention relates to planar optical waveguides and a methodof making the waveguides.

BACKGROUND

[0002] Optical fiber communication systems are becoming prevalent. Theoptical fiber system can comprise an optical fiber, and a wide varietyof other optical devices for receiving, transmitting, and using signals.Optical waveguides, which provide a means of receiving and processingsignals from optical fibers, have proven to be useful devices in opticalfiber systems.

SUMMARY OF INVENTION

[0003] The present invention features a novel optical waveguidecomprising an optical confinement layer. The waveguide has a lowercladding layer, a core layer, a patterned optical confinement layerproximate to the core layer, an optional upper cladding layer, and anoptional substrate. The optical confinement layer may comprise, e.g.,MgF₂, LiF, or any other low index material. The core layer may comprisea high index silica doped with, e.g., aluminum, titanium, tantalum,zirconium, germanium, hafnium,or phosphorus. Alternatively, the corelayer may comprise silicon oxynitride (SiON) or silicon nitride (Si₃N₄).The cladding layers may comprise silica. For example, the lower claddinglayer may comprise a low index silica that may be doped with boron orfluorine. The substrate may comprise silicon or silica. Alternatively,the cladding layers and core layer may each comprise a polymer. In someembodiments, the waveguide may include layers doped with rare earthelements, such as erbium.

[0004] The location of the optical confinement layer may vary as long asit is proximate to the core layer. For example, the optical confinementlayer can be between the lower cladding layer and core layer, betweenthe upper cladding layer and core layer, embedded in the core layer,embedded in the upper cladding layer, or embedded in the lower claddinglayer.

[0005] As used in this invention:

[0006] “optical confinement layer” means a layer in an optical waveguidedevice, distinct from the core layer and the cladding layer, having arefractive index lower than that of the cladding layer, and patterned soas to laterally guide light in the waveguide; and

[0007] “lateral” directions refer to directions in the plane of theoptical confinement layer.

[0008] An advantage of at least one embodiment of the present inventionis ease of manufacture. In some embodiments, this invention allowsfabrication of waveguides using simple photolithography and lift-offpatterning techniques, eliminating the need for reactive ion etching(and the associated equipment costs), and minimizing the number ofprocessing steps required.

[0009] Another advantage of at least one embodiment of the presentinvention is low optical loss. In some embodiments, this invention usesvery thin layers for lateral optical confinement layers. Because of thethinness of these layers, they may be patterned smoothly and preciselyusing lift-off or etching techniques, thereby minimizing optical lossesdue to scattering.

[0010] Another advantage of at least one embodiment of the presentinvention is that the resultant device can have a substantially planarsurface, which can be useful for applications in which that surface isto be bonded to a submount. Such applications include those in which thewaveguide is to be aligned with another device.

[0011] Other features and advantages of the invention will be apparentfrom the following drawings, detailed description, and claims.

BRIEF DESCRIPTION OF DRAWINGS

[0012]FIG. 1 depicts a waveguide of the present invention having theoptical confinement layer between the core layer and lower claddinglayer.

[0013]FIG. 2 depicts the waveguide of FIG. 1 with an optional uppercladding layer.

[0014]FIG. 3 depicts a waveguide of the present invention having theoptical confinement layer between the core layer and upper claddinglayer.

[0015]FIG. 4 depicts a waveguide of the present invention having theoptical confinement layer embedded in the core layer.

DETAILED DESCRIPTION

[0016] Optical waveguides typically consist of a substrate layer, alower cladding layer, a core layer, and, optionally, an upper claddinglayer. They may also comprise a substrate. The present inventionprovides an optical waveguide having a patterned optical confinementlayer proximate to the core layer. The present invention may be realizedwith a number of different materials systems, including silica glasses.Construction of the various layers of the waveguide can be accomplishedby known techniques appropriate for the specific materials. Thesetechniques include chemical vapor deposition (for example, atmosphericpressure chemical vapor deposition, low pressure chemical vapordeposition, plasma enhanced chemical vapor deposition, or metal-organicchemical vapor deposition), sputtering, vacuum evaporation, oxidation,spin-coating (including sol-gel techniques), flame hydrolysisdeposition, and other techniques. FIG. 1 depicts an optical waveguide 10of the present invention.

[0017] The substrate 12 is preferably a semiconductor, such asmonocrystalline silicon, and it preferably has two substantially planarmajor surfaces, at least one of which is preferably optically smooth.Alternatively, the substrate may be ceramic or glass. If desired, thesubstrate could be removed, by etching, after the waveguide is built.

[0018] The lower cladding layer 14 is applied onto the substrate by anytechnique appropriate for the specific material used. The lower claddingmay comprise, for example, undoped or doped silica, and may be formed,for example, by plasma-enhanced chemical vapor deposition (PECVD), or inthe case of a silicon substrate, by thermal oxidation of the substrate.The lower cladding layer typically isolates the fundamental optical modefrom the substrate. Of course, the lower cladding layer preferablyexhibits low absorption and low scattering of light at the wavelengthsof interest. In principle, if the substrate has the required opticalcharacteristics, the substrate itself may serve as the lower claddinglayer.

[0019] The core layer 16 is applied onto the lower cladding layer by anytechnique appropriate for the specific material used. Useful corematerials for waveguides have an index of refraction that is higher thanthat of the lower cladding layer. For example, in the case of a Sisubstrate and undoped silica lower cladding, the core material may besilicon oxynitride (see, for example, “Silicon Oxynitride Layers forOptical Waveguide Applications,” R. Germann et al., Journal of theElectrochemical Society, 147 (6), pp. 2237-2241 (200)), silicon nitride,or other high index doped silica glasses (e.g., silica doped with Al,Ge, P, Ti, Ta, Hf, or Zr).

[0020] As illustrated by FIG. 2, in some embodiments, an upper claddinglayer 18 may be formed onto the core layer. The upper cladding layershould have an index of refraction lower than that of the core layer.The upper cladding layer may be conveniently made of the same materialas the lower cladding layer (although other materials with appropriateoptical properties may be used).

[0021] Pursuant to the present invention, a patterned opticalconfinement layer 20 is formed proximate to the core layer. In general,the optical confinement layer is located within the optical mode of thewaveguide. Therefore, the optical confinement layer may be locatedbetween the core layer and lower cladding layer, as shown in FIGS. 1 and2, between the core layer and the upper cladding layer, as shown in FIG.3, within the core layer, as shown in FIG. 4, or even within the upperor lower cladding layer (not shown) so long as it is within theevanescent tail of the optical mode.

[0022] The optical confinement layer may comprise any patternablematerial with sufficiently low refractive index, including MgF₂, LiF, orany other low index material such as low index doped silica (forexample, silica doped with boron or fluorine). Preferably, it comprisesMgF₂, and has a thickness greater than 10 nm and less than 500 nm. Theoptical confinement layer may be patterned by standard photolithographymethods, using either lift-off or etching techniques. In someembodiments in which the optical confinement layer is designed to be avery thin layer (e.g., less that 100 nm) of a material that may bevacuum deposited (e.g., MgF₂) the lift-off technique is particularlyconvenient for forming high-resolution patterns. Alternatively, etchingtechniques include, but are not limited to wet chemical etching,reactive ion etching, and ion beam etching. The pattern of the opticalconfinement layer can define the waveguide for a variety of opticaldevices, e.g., straight waveguides, curved or tapered waveguides,couplers, Mach-Zehnder devices, and optical amplifiers.

[0023] Appropriate compositions and thicknesses of the core layer, lowercladding layer, upper cladding layer, and optical confinement layer aredesigned through numerical modeling. One well-known waveguide modelingtechnique is the “transfer-matrix” approach (see, for example,Guided-Wave Optoelectronics, Theodor Tamir (Ed.), 2nd Edition,Springer-Verlag). Alternatively, commercial waveguide modeling tools maybe used, including OptiBPM, from OptiWave Corporation, Ottawa, ON,Canada.

[0024] The effectiveness of an optical confinement layer in defining thelateral extent of the waveguide may be understood in terms of an“effective index” model. In the regions where the optical confinementlayer exists, the effective index of the slab mode is lowered, comparedto regions where there is no confinement layer. Therefore, asillustrated in FIG. 1, a lateral waveguide is formed, with the lateral“core” 22 being the region not having the confinement layer and thelateral “cladding” 24 being the region in which the confinement layer islocated.

[0025] The choice of materials used for the substrate and various layersof the waveguide depends largely on the wavelength range of the light tobe guided and the ultimate application. In addition to the materialsalready mentioned, a wide range of materials may be used, including, butnot limited to polymers, polycrystalline materials, and non-silicaglasses (for example, bismuth oxide, zirconia, chalcogenide glasses andfluoride glasses). It is also possible to make a waveguide in which thelower cladding layer, core layer, and, if present, the upper claddinglayer each comprise a polymer.

[0026] The optical waveguides of the present invention are suitable foruse as planar waveguides, Wavelength Add-Drop (WAD) for Dense WavelengthDivision Multiplexing (DWDM) systems, Er-doped optical amplifiersystems, Mach-Zehnder (MZ) based switches, tunable filters, Y-branchswitches, and optical amplifiers.

[0027] This invention may be illustrated by way of the followingexample.

EXAMPLE

[0028] Silicon Oxynitride Waveguide

[0029] In this example, the tapered waveguide was formed by startingwith a (100) silicon wafer, polished on both sides. An oxide was grownon the top surface of the wafer to a thickness of at least 3 μm, formingthe lower cladding. Using standard techniques, photoresist (NR7-1000PY,available from Futurrex, Inc., Franklin, N.J.) was patterned on thelower cladding, defining the desired waveguide pattern. In this example,the straight waveguide was 7.5 μm wide. The wafer was placed in a vacuumdeposition system, and a thin layer of MgF₂ (the optical confinementlayer) was deposited by electron-beam evaporation (alternatively, otherdeposition techniques would work, including thermal evaporation, orsputtering). After being removed from the deposition system, thephotoresist was removed in acetone (other appropriate solvents could beused), thereby patterning the MgF₂ by lift-off. In this example, theMgF₂ was 37.5 nm thick. Next, the wafer was placed in a plasma-enhancedchemical vapor deposition (PECVD) system. The silicon oxynitride corewas deposited, followed by the silica upper cladding. In this example,the core had a thickness of 0.4 μm and the nitrogen content in the corewas adjusted by controlling the PECVD gas flows to provide a refractiveindex of 1.668 at the design wavelength of 1480 nm. The upper claddinghad a thickness of 1.64 μm. The resulting waveguide structure provided avery nearly planar surface (suitable for bonding to a submount) andsupported a single transverse mode at the design wavelength of 1480 nm.

[0030] The waveguide absorption loss could be minimized by a finalhigh-temperature (e.g., 1140° C.) anneal to reduce hydrogen in the PECVDlayers.

[0031] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and spirit of this invention and it should be understood that thisinvention is not to be unduly limited to the illustrative embodimentsset forth herein.

What is claimed is:
 1. An optical waveguide comprising: a lower cladding layer, a core layer, and a patterned optical confinement layer proximate to the core layer.
 2. The optical waveguide of claim 1 further comprising an upper cladding layer.
 3. The optical waveguide of claim 1 wherein the optical confinement layer comprises MgF₂.
 4. The optical waveguide of claim 1 wherein the optical confinement layer comprises LiF.
 5. The optical waveguide of claim 1 wherein the optical confinement layer comprises a low index doped silica.
 6. The optical waveguide of claim 1 wherein the core layer comprises a high index doped silica.
 7. The optical waveguide of claim 6 wherein the silica is doped with a material selected from the group consisting of aluminum, titanium, tantalum, zirconium, germanium, hafnium, and phosphorus.
 8. The optical waveguide of claim 1 wherein the core layer comprises silicon oxynitride or silicon nitride.
 9. The optical waveguide of claim 2 wherein both cladding layers comprise silica.
 10. The optical waveguide of claim 5 wherein the silica is doped with boron or fluorine.
 11. The optical waveguide of claim 1 wherein the lower cladding layer and core layer each comprise a polymer.
 12. The optical waveguide of claim 2 wherein the lower cladding layer, upper cladding layer, and core layer each comprise a polymer.
 13. The optical waveguide of claim 1 further comprising a substrate.
 14. The optical waveguide of claim 13 wherein the substrate layer comprises silicon.
 15. The optical waveguide of claim 1 wherein the core layer is doped with a rare earth element.
 16. The optical waveguide of claim 1 wherein the optical confinement layer is between the lower cladding layer and core layer.
 17. The optical waveguide of claim 2 wherein the optical confinement layer is between the upper cladding layer and core layer.
 18. The optical waveguide of claim 1 wherein the optical confinement layer is embedded in the core layer.
 19. The optical waveguide of claim 2 wherein the optical confinement layer is embedded in the upper cladding layer.
 20. The optical waveguide of claim 1 wherein the optical confinement layer is embedded in the lower cladding layer. 